RADIATION PROTECTION AND SAFETY FROM NUCLEONIC GAUGES

                           Ionizing radiation can be very hazardous to humans and steps must be taken to minimize the risks. This Section provides only a brief summary of some of the principles of radiation protection associated with the use of sources of ionizing radiation used in nucleonic gauges. In order to concentrate on the important principles a certain fundamental level of knowledge of radiation physics has been assumed. An explanation of quantities and units is available in other IAEA publications

                          The essential requirements for protection from ionizing radiation are specified in the International Basic Safety Standards (BSS). The Standards state that the prime responsibility for radiation protection and safety lies with the Licensee, Registrant or employer. Some of the fundamental requirements of the Standards relevant to nucleonic gauges are discussed in this section, but the Standards should be consulted in full for a comprehensive understanding of their requirements.

Principles of dose limitation:
The principles of dose limitation are briefly summarized below

• 1.no application of radiation should be undertaken unless justified,
• all doses should be kept “as low as reasonably achievable” (ALARA), economic and social factors being taken into account,
• in any case, all individual doses must be kept below dose limits.

            It should be emphasized that the most important aspect of dose limitation, assuming that the practice is justified, is to keep radiation doses As Low As Reasonably Achievable (ALARA).

Dose limits: 

                   The dose limits for workers and the public are given below, although doses to gauge operators

are expected to be significantly below these levels during normal operations.

Occupational dose limits:

                  Occupational dose limits are chosen to ensure that the risk to radiation workers is no greater than the occupational risk in other industries generally considered safe. Radiation doses must always be kept as low as reasonably practicable, but some industries may require employees to routinely work in high radiation areas and therefore dose limits are required. The BSS specifies that doses to individuals from occupational exposure should not exceed:

• an effective dose of 20 mSv per year averaged over 5 consecutive years
• an effective dose of 50 mSv in any single year
• an equivalent dose to the lens of the eye of 150 mSv
• an equivalent dose to the extremities (hands or feet) or the skin of 500 mSv in a year.

Public dose limits:

                  If the use of nucleonic gauges may lead to the public being exposed, then the following dose limits must not be exceeded.

• an effective dose of 1 mSv in a year
• in special circumstances, an effective dose of up to 5 mSv in a single year provided that the average dose over five consecutive years does not exceed 1 mSv per year.
• an equivalent dose to the lens of the ye of 15 mSv in a year
• an equivalent dose to the skin of 50 mSv.

Authorization: 

                 In order to control the use of radiation sources and to ensure that the operating organization meets the requirements of the BSS, the legal person responsible for any radiation source will need to apply for an authorization from the national Regulatory Authority. This authorization is usually in the form of a license or registration. Prior to buying or acquiring a nucleonic gauging system, the operating organization will, therefore, need to apply for such an authorization from the regulatory authority. The regulatory authority will need details about the gauging equipment, such as: the purpose for which it will be used, the radionuclide(s) and activity, manufacturer and model, details of the storage facility and installation site, copies of approval certificates, end of life considerations (disposal or return to supplier) etc. The regulatory Authority will also need: information regarding the people who will be using the equipment, such as their qualifications and training in radiation safety etc. .Further details about the relevant legal and governmental infrastructure, the regulatory control of sources, and the notification and authorization for the possession and use of radiation sources are available from IAEA.

Inspection and enforcement:

                  The Regulatory Authority may inspect the registrant/licensee to audit their provisions for radiation safety and to physically inspect the premises. Enforcement action may be taken against the operating organization if the level of radiation protection and safety are considered unacceptable.

                 IAEA have published a ‘Categorization of Radioactive Sources’ which provides a relative ranking of radioactive sources in terms of their potential to cause severe deterministic effects (i.e. how ‘dangerous’ they are). The Categorization is composed of 5 Categories — with Category 1 sources being the most ‘dangerous’ and Category 5 the least ‘dangerous’. Gauges generally fall into categories 3 and 4.

Practical protection for gauge users:

                  The practical elements to radiation protection are: Time, distance, shielding and prevention of access. These are discussed in detail below.

Time: 

                  Radiation is normally emitted from a source at a constant rate and this is measured in microsieverts per hour (μSv/h) or millisieverts per hour (mSv/h). The shorter the time a person spends in the radiation field the lower the radiation dose will be to that individual. It is therefore advisable not to linger in areas where there may be high radiation levels and any work done close to a source should be done efficiently. This will help to ensure that the radiation risks are kept as low as reasonably achievable.

Distance: 

                  Radiation levels decrease rapidly with increasing distance and it is therefore important to never directly handle radiation sources. Specially designed tools with long handles must always be used if a source is to be replaced or manipulated.

Shielding: 

                 The main consideration for gauges is to prevent access to the high radiation levels close to the source. This can be achieved by providing an adequate thickness of suitable shielding material around the source. The amount of shielding required will be determined by the type and energy of the radiation and the activity of the source. For example several centimeters of lead may be required around a gamma source or a several millimeters of aluminium around a beta source. The environment in which the gauge will be used should also be considered when deciding on the material and design of the shielding (e.g. high temperature or corrosive chemicals could significantly reduce the effectiveness of the shielding).

Prevention of access:

               In many cases it is not possible to fully shield the source and the material to be examined. It will, therefore, be necessary to prevent access to any areas of high radiation by using shutters (manual or automatic), mechanical guarding or interlock systems. In some cases the designation of controlled areas may be additionally required in order to restrict access to authorized persons only.

Warning notices: 

               All radiation sources should display the radiation trefoil to warn of the potential hazard. Details of the radionuclide, activity on a specified date and serial number should be included on a label permanently attached to the source housing. Any shutters should be clearly marked to indicate the status of the source to persons in the vicinity. X ray equipment should also display a clear indication when radiation is being generated. Notices should state whether any controlled areas are designated around the gauge.

Radiation monitoring: 

             Operating organizations need to have in place an effective programme for monitoring occupational exposure to radiation. Guidance on establishing a monitoring programme for external exposure, the appropriate dosimetry to be used for workplace and individual monitoring and record keeping is given in an IAEA Safety Guide.

Workplace monitoring:
             Portable dose rate monitors can be used to measure radiation levels (normally in microsieverts or millisieverts per hour) around gauges. Monitoring may be carried out for several reasons, for example to:

• check the shielding around a gauge is intact
• check a shutter is closed before carrying out maintenance on or close to a gauge
• check the radiation levels around a shipping container to ensure it is safe to transport
• confirm the extent of a controlled area around a gauge
• check the shielding around a source storage facility is acceptable.

Storage:

              There will be occasions when sources need to be stored. For example, portable gauges not in use, gauges removed from a production line during maintenance, old gauges awaiting disposal, etc. To ensure the safety and security of the sources the storage facilities should:

• provide adequate shielding,
• be physically secure (e.g. locked when not in use)
• not be used as a general storage area for other goods
• be fire proof and not contain other hazardous materials (e.g. flammable liquids) be dry
• appropriately labeled (e.g. radiation trefoil and warning notices in a local language).

Source accountancy:

               Records need to be kept which show the location of each source at all times. National regulations may specify how frequently the accountancy checks need to be carried out, but in general, the following can be applied

• sources in permanently installed gauges should be accounted for at least once per month
• sources in portable gauges should be accounted for every day they are out of the store and once a week when they are in storage.

Maintenance:

               Nucleonic gauges are often used in harsh environmental conditions which may result in the radiation safety and protection of the gauge be adversely affected, for example; shielding may be degraded, shutters may stick, warning notices may become illegible, etc. It is therefore important that gauges are included in a routine maintenance schedule. Persons carrying out the maintenance work need to be aware of the radiation hazards and be appropriately trained. When working close to a gauge a radiation monitor should always be used to confirm that any shutters are fully closed and that the source is fully shielded.

Leak testing:

               When a new radioactive source is purchased it should be supplied with a certificate confirming that it is free from contamination. Periodic re-checks need to be carried out by an appropriately trained and qualified person to ensure that the structure of the source remains intact. Gauges that are used under harsh environmental conditions (e.g. high temperature, corrosive chemicals, and high levels of vibration) may need to be checked more frequently. The intervals for leak testing should not normally exceed 2 years (and may be more frequent), but this will normally be specified by the regulatory authority.

              Many accidents have occurred with disused or abandoned sources. Before a source is purchased, consideration needs to be given to what will happen to the source when it is no longer of use or if the operating organization goes bankrupt etc. In many cases the preferred option is to return the source to the supplier, possibly for recycling. Other options include permanent disposal or long-term storage. All options have financial and logistical consequences that need to be considered before the gauge is purchased.